More precisely, the aim of the invention is to provide a particular embodiment of the detection electrodes for such matrix detectors.
There are various types of radiation detectors, including, conventionally, gas detectors, scintillator detectors and semiconductor detectors. The latter detectors have the advantage of possessing a high atomic number, allowing a maximum number of incident photons to be absorbed for a minimum material thickness.
Furthermore, they also have the dual advantage of converting the photon signal directly into an electrical signal, without prior conversion into a light signal, and of having a relatively wide bandgap. In other words, semiconductor detectors therefore allow effective conversion of radiation into electrical signals with a small material thickness (a few millimeters).
The semiconductors more particularly chosen are of the II-VI type, such as for example CdZnTe, CdTe, Cl:CdTe, Cl:CdTeSe, Cl:CdZnTe, In:CdTd and In:CdZnTe.
These cadmium/tellurium-based semiconductors may especially be obtained, for example, by a Bridgman or THM growth method and they have, as major characteristics, a very high electrical resistivity (108–1011 ∩·cm) essential for producing an X-ray or γ-ray spectrometer dedicated more particularly to medical, industrial or scientific imaging.
The actual detector therefore comprises a photosensitive material, consisting of a parallelepiped made of a semiconductor material, conventionally cut out from an ingot slice, resulting from the production of said semiconductor, and it has at least two parallel faces on which the electrical contacts, for biasing and for receiving the electrical signals generated by the incident radiation, are produced.
For example, one of the faces receives a full-face bias electrode, and the opposite face is intended to receive a plurality of what are called detection electrodes, these being distributed in a matrix form and defining the pixels for the location of the point of impact of the incident radiation on the detector.
Other architectures may be envisioned; in particular, the face receiving the radiation is not necessarily coated with an electrode.
To be able to have the most effective detector possible, the electrical contacts produced on the electrodes, especially the detection electrodes, must not modify the behavior of the device and must consequently have a negligible resistance to the current flow, in comparison to that of the material. These contacts must be of the ohmic type, that is to say they must have an almost linear current-voltage characteristic, advantageously use judicious band bending at the metal/semiconductor contact, exhibit a tunnel effect at this point, and undergo recombination in the space charge region.
The major difficulty therefore lies in how to produce these ohmic contacts on semiconductor materials, especially of the II-VI type, as, apart from the appropriate electrical behavior that it is necessary to obtain, the electrodes must then be connected, for example, to a read circuit.
These difficulties are therefore based on various problems involving the materials employed, apart from the increasingly reduced size of the detection electrodes.
Firstly as regards the electrodes, it is difficult to produce an ohmic contact with a high-resistivity material, of the CdTe type (and its compounds), owing to the high work function (5.02 eV) of CdTe. In fact, only platinum and gold can be employed. Such contacts are produced by evaporation or sputtering, and are neither ohmic nor blocking, but lie between the two.
At the same time, other physico-chemical phenomena, such as the surface states before deposition of the metal or oxidation of the surface, fix the height of the potential barrier independently of the work function of the metal.
As a result, it is therefore possible to obtain an ohmic contact if the carriers can flow freely by a tunnel effect.
This mode of transport is favored by electrochemical deposition of metal from gold chloride (AuCl3) or platinum chloride (PtCl4) solutions on a surface that has been chemically etched beforehand.
The metal is chemically reduced by tellurium and acts as a strong accepter dopant on the surface of the detector (see the 1985 publication by J. P. Ponpon, Solid State Electronics, Vol. 28, No. 7, 689, (1985); and J. Phys., Applied Physics 16(83), 2333–2340 by Janick and Triboulet—CNRS).
This metal deposited by electrochemical deposition (also called electroless plating), from gold or platinum chloride, takes the place of cadmium on the surface of the detector, said cadmium enriching the electrochemical solution.
The maximum thickness deposited owing to the polarization effect and to the intrinsic strains in the layer is in general less than 500 Å. Furthermore, the adhesion of the contact depends on the preparation of the surface, on the chemistry of the metal/semiconductor interface, on the thickness of the deposit, but also on its area.
Now, the tendency in for example the field of nuclear detection is to go toward producing pixelated monolithic detectors in which the area of each individual electrode will tend, long term, toward 50 μm2, whereas at the present time it is a few square millimeters. Adhesion therefore constitutes a critical parameter, in particular during the assembly operations.
This adhesion is therefore of paramount importance, the more so as the materials present do not have the same expansion coefficients. Temperature variations consequently induce strains.
Furthermore, the ohmic contact generally produced on these materials has the drawback inherent in the small thickness of the electrode which, as already mentioned, is typically less than 500 Å.
Consequently, the connection to a read circuit proves to be difficult and generally requires the metal deposit constituting the electrode to be thickened, so as to reach a few thousand ångströms at said electrode, especially in order to guarantee a quality connection.
Now, the deposition of such a thickening layer, by vacuum deposition or by the addition of microdrops of conductive adhesive, also poses a problem since it is not uncommon for such a layer to overhang beyond the actual electrode, capable of impairing the precision of the final detection obtained.
Another difficulty lies in the regions separating each of the detection electrodes, called hereafter interpixel regions.
Specifically, to end up with correct operation of the detector, especially in terms of precision, the electrical resistance between each pixel, that is to say between each detection electrode, must be a maximum and typically similar to the intrinsic resistance of the detector material, and typically a few gigaohms, so as to avoid interactions between pixels.
It is possible to maintain such a high resistance only by avoiding any contamination between the pixels, this being conventionally achieved by protecting the corresponding surface of the detector, either by passivation or by encapsulation of an inert and perfectly impermeable material.
However, the deposition of such layers, particularly encapsulation layers, is tricky, especially for such small dimensions, since these layers generally are of an organic nature, and not etchable.